Kinetic and Equilibrium Function and Switchable Catalytic Activity of Some Thermo-Responsive Hydrogel Metal Absorbents Based on Modified PNIPAM

Synthesis and identification of some thermo-responsive hydrogel metal absorbents based on Schiff base modified PNIPAM is described. PNIPAM hydrogel is produced through conventional free-radical polymerization method. The hydrogel is amino modified with different amines and then treated with various aldehydes to make polymeric Schiff-bases supposing to utilize as an absorbent for Cu salt. Characterization of the as-prepared hydrogels was performed properly and confirmed the corresponding structures. Kinetic and equilibrium function of the absorbents and the effect of distinct variables like pH, adsorbent amount and contact time in the absorption process are investigated in detail. Kinetic and adsorption isotherm data agreed with pseudo-second order model and Langmuir isotherm, respectively. The maximum value of adsorption capacity of the prepared adsorbent was around 8550 mg g−1. In addition, the corresponding switchable catalytic activity using one of the thermo-responsive hydrogel metal absorbents in the reduction of 4-nitro phenol to 4-amino phenol in the role of model reaction is examined.


Introduction
Stimuli-responsive or smart polymers undergo an abrupt change in their physical or chemical properties concerning to small external stimuli like temperature [1], pH [2], magnetic or electric field [3], light intensity [4], biological molecules [5] and ionic strength [6].These environmental stimuli produce a significant macroscopic and reversible alteration in the structure of smart polymers which include change in physical state, solubility, shape, conductivity, solvent interactions, variation of the hydrophilic-lipophilic balance and delivery of therapeutic agents [7][8][9].Hydrogels are one type of these intelligent polymers which can be described as a cross linked 3-D network of hydrophilic synthetic or natural polymers.In fact, they have the potential to reversible swelling or de-swelling in the presence of water and retain extensive amount of water in the swoll en condition.Smart hydrogels that make a response instantly to the environmental changes have been widely used in drug delivery [10], biosensor [11], wound healing [12], bioseparation [13], tissue engineering [14] and environmental applications like adsorption [15].Smart hydrogels are categorized based on the type of the stimuli that caused their swelling behavior.The most important class of smart hydrogels are: pH-sensitive or ion-sensitive, temperature-sensitive, photo/ electric-sensitive, enzyme-sensitive [16] and dual or multi stimuli-sensitive hydrogels [17].Temperature-sensitive hydrogels are one of the most promising hydrogels because they form a gel above a certain temperature and change to a liquid state at a lower temperature.This threshold is called the lower critical solution temperature (LCST).At temperatures lower than LCST, the hydrogel is in the "sol" state in all proportions.Whereas above LCST, it becomes hydrophobic and insoluble which leads to "gel" formation and phase separation occurs [7].The presence of hydrophilic functional groups in the polymer chain increases the LCST, while hydrophobic groups lower this temperature [18].Poly(N-isopropylacrylamide) (PNIPAM), poly(N,N-diethylacrylamide), poly(N-vinylalkylamide) and poly(N-vinylcaprolactam) are generally applied polymers for the synthesis of temperature-responsive hydrogels [19].Apart from biological use of stimuli-responsive hydrogels, they have been widely employed as a novel absorbent in environmental instances which is the main purpose of this study.
The discharge of copper ions at high concentrations into the effluent is a major trouble came across by many industries such as electroplating [20], metal finishing [21], textile [22] and battery manufacturing [23].Extreme consumption of copper may lead to problems such as nausea, diarrhea, respiratory difficulties, headache, vomiting, and kidney & liver failure [24].Therefore, copper-polluted wastewater must be refined prior to discharging it to the environment.Because of the harmful effect of copper on humans and living organisms, World Health Organization (WHO) has reported the permissible amount of Cu ions in drinking waters as 2 mg/L.Several methods like adsorption, chemical precipitation, ion exchange, electrodialysis, reverse osmosis, membrane filtration and neutralization have been vastly used to remove copper ions from water and wastewaters [25].Most of these methods are very costly and usually produce sludge or waste products that are difficult to dispose.Adsorption is an effective process for eliminating copper ions because of its cost effectiveness, high selectivity, simplicity, low sludge production and abundance of various adsorbents.Recently, different polymers have been applied as an adsorbent to capture Cu ions from water.These macromolecular adsorbents are typically copolymerized or grafted with metal-chelating agents like amide, pyridine, carboxyl and imidazole.PNIPAM hydrogel is becoming a widely known adsorbent for removing different metal ions due to its phase transition behavior.It is a temperature-sensitive hydrogel that changes from fully hydrated PNIPAM to collapsed state at a temperature greater than its LCST.Adding cross-linking agents or inclusion of co-monomers can create cross-linked PNIPAM-based hydrogels which undergo reversible conformation changing over its volume phase transition temperature.
Many researchers have focused on the development of novel catalysts with the possibility to switch their activity in different chemical processes by an external stimulus.These artificial switchable catalysts have smart features allowing them to change from an active state "on" to an inertness state "off" by applying an external stimulus such as temperature, pH, light and electric or magnetic fields.As temperature is the most widely investigated stimuli, different temperaturesensitive polymers have been introduced as temperaturecontrolled switchable catalysts.In most cases, an artificial switchable catalyst based on PNIPAM has been developed which faces a reversible phase transition at its LCST (32 °C) [26].Tzounis et al. [27] prepared a temperature-controlled catalyst consisting of core-shell AuAg NPs enclosed in PNI-PAM microgels with silver satellites incorporated into the microgel structure and used them to catalyze the formation of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP) using NaBH 4 as a reducing agent.Kakar et al. [28] incorporated Cu, Pd and Cu/Pd NPs into thiol modified PNIPAM microgels.They showed that PNIPAM-Cu/Pd is an efficient catalytic system for the hydrogenation of methylene blue and 4-NP.Chen et al. [29] designed PNIPAM/graphene oxide (GO)-Ag hydrogel by incorporating GO-Ag composite into N-isopropylacrylamide (NIPAM).They reported that this hydrogel is an effective catalyst for the reduction of 4-NP to 4-AP.Li et al. [30] prepared Fe(0) NPs and incorporated them on a temperature-responsive hydrogel PNIPAMpoly(hydroxyethyl) methacrylate (PHEMA).This hydrogel (Fe0@PNIPAm-PHEMA) was very effective catalyst in the hydrogenation of 2-NP, 3-NP, and 2-chloro-4-nitrophenol to their corresponding amino reduce compounds.This hydrogel has prospective potential for reductive degradation of nitrophenol contaminants.
In this study, a thermo-responsive hydrogel based on PNI-PAM was prepared using free radical polymerization and subsequently was modified with different diamines.Then, amino modified hydrogels were added to different aldehydes to produce Schiff-base modified PNIPAM hydrogels for removing Cu ions from aqueous solutions.Finally, several isotherm models were applied to assess the equilibrium adsorption and three kinetic models were explored to study the adsorption kinetics of Cu ions on the prepared hydrogel.The switchable catalytic activity of one of the thermoresponsive hydrogel Cu absorbents in the reduction of 4-NP to 4-AP is investigated.Temperature responsiveness feature of the catalyst causes the catalyst's switchable on/off behavior below and above the LCST of smart catalyst.
FT-IR data were collected on a FT-IR 8300 spectrophotometer, Shimadzu.The IR disks were prepared by grinding the samples with KBr salt and subsequent pressing.Cu amounts on polymers were obtained by Varian Vista-Pro ICP-OES apparatus through acid digestion of samples.Chemical composition of samples include C, N and H amounts were determined by CHN elemental analyzer, 1112 series thermo finningan FIASHEA.Scanning electron microscopy (SEM) micrographs were obtained with VEGA3 TESCAN apparatus at 20 kV.The freeze-dried specimens were sputtered in the process of applying an ultra-thin coating of Au using KYKY SBC12 sputter coater before SEM imaging to produce better quality depictions.

Adsorbent Preparation
Preparation of Hydrogel Based on PNIPAM Using Free Radical Polymerization (I) NIPAM (monomer, 8.83 mmol, 1.00 g), NNMBA (crosslinking agent, 0.089 mmol, 0.013 g) and AIBN (initiator, 0.044 mmol, 0.0072 g) were dissolved in 1, 4-dioxane (2 mL, freshly distilled from sodium wire under N 2 ).The reaction mixture was stirred under inert atmosphere at 80 °C for 2 h.The resultant 1% cross-linked PNIPAM hydrogel was soaked in water for 1 day to remove unreacted monomers or other unconsumed starting materials, and was dried under vacuum at 60 °C overnight.

Preparation of Amino Modified PNIPAM Hydrogels (II, III and IV)
The PNIPAM hydrogel (I) was added to different diamines (hydrazine, ethylenediamine and hexamethylenediamine) (fivefold with respect to NIPAM amounts).The reaction mixture was stirred at 90 °C for 9 h.Afterward, the amino modified PNIPAM hydrogels (II, III and IV) were soaked thoroughly in distilled water around 2 days to eliminate unreacted amines and then dried under vacuum at 60 °C overnight.

Preparation of Schiff-Bases Modified PNIPAM Hydrogels (V, VI and VII)
Amino modified hydrogel (III) (0.9 g) was added to different aldehydes (salicylaldehyde, pyridine-and thiophene-2-carbaldehyde) (2 eqv.toward amine content) in EtOH (2 mL).The mixture was heated at 60 °C for one day while stirring.The resulted products (V, VI and VII) were soaked thoroughly in ethanol totally for 2 days to eliminate unreacted aldehydes and then dried under vacuum at 60 °C overnight.

Adsorption Isotherm Models
The copper adsorption capability of the prepared sorbents were investigated in aqueous solutions of copper (II) acetate monohydrate Cu(OAc) 2 .H 2 O.We examined the removal of copper salt by the as-prepared sorbents in batch experiments at room temperature.Typically, Schiff-base modified PNIPAM hydrogels (V, VI and VII) (0.01 g) were added to 2 mL copper ion solution and shaked for 24 h at ambient temperature, with specified primary concentrations changed from 2.5 up to 20,000 mg L −1 (2.5, 10, 50, 100, 250, 500, 800, 1500, 2000, 2500, 3000, 5000, 7000, 10,000, 15,000, 20,000).After reaching an equilibrium, the copper binding hydrogels (VIII, IX and X) were isolated by filtration and the filtrates that contained the residual concentration of the Cu 2+ solution were analyzed by ICP to determine the equilibrium concentration of the copper ion (adsorbate) in solution (C e , mg L −1 ).
The removal efficiency of Cu can be calculated by the subsequent equation: In Eq. ( 1), the removal efficiency of Cu 2+ and the Cu 2+ concentrations (mg L −1 ) at beginning and at equilibrium, are R e , C 0 , and C e , respectively.
Afterward, the equilibrium adsorption capacity of the synthesized sorbents, q e (mg copper/g sorbent) was measured through the following equation: In Eq. ( 2), q e is the equilibrium adsorption capacity (mg g −1 ), V is the primal volume of the Cu 2+ solution (mL), M is the adsorbent mass (g), and C 0 and C e are initial and equilibrium concentrations of Cu 2+ (mg L −1 ).
Three adsorption isotherms i.e.Langmuir, Freundlich and Temkin isotherm models are employed to investigate the adsorption mechanism of Cu 2+ ions onto Schiff-bases modified PNIPAM hydrogels using the data of adsorption.
The linear Langmuir model can be described through the following equation: where q m is the greatest amount of uptake capacity (mg g −1 ), K L is the adsorption constant for Langmuir isotherm (L mg −1 ), reflecting the tendency of the absorbate to the absorbent or adsorption energy.In addition, q e is the adsorption capacity (mg g −1 ) and C e is the concentration of Cu 2+ ions at equilibrium point (mg L −1 ) as defined before.
Whereas the Freundlich equation is illustrated by the following equation: wherein q e is the equilibrium adsorption capacity (mg g −1 )  and C e is the equilibrium concentration of the Cu 2+ (mg L −1 ), as stated before.Furthermore, K F is the Freundlich constant (mg g −1 ) (L mg −1 ) 1/n which has a positive affinity with the adsorption capacity and n is the heterogeneity factor and associated with the grade of heterogeneity and is appropriate to the quantity of the adsorption driving force.
High n values accordingly demonstrate a fairly homogenous surface, while low n values express too much adsorption even at low solution concentrations and reveal the presence of high distribution active sites with high energy.
The Temkin adsorption isotherm is illustrated by the subsequent equation: where, A T (L mg −1 ) and B = RT/b T (J mol −1 ), are Temkin constants, R (8.314 J mol −1 K −1 ) is the ideal gas constant and T (K) is the temperature.

Adsorption Kinetic Models
In order to investigate the impact of contact time on the adsorption of Cu(II) by hydrogels, kinetics tests were carried while solutions concentration, temperature, absorbent dosage and pH of initial salt solution were kept constant in the amount of 10 mg L −1 , 25 °C, 0.01 g and 6 respectively.The sorbent (V) was treated with aqueous solution of Cu(OAc) 2 .H 2 O (2 mL, 10 mg L −1 ) at pH-value = 6 and shaked at ambient temperature for 24 h.At varied time intervals (0.5, 1, 2, 3, 4, 5, 6, 12 and 24 h), hydrogels were isolated by filtration and the filtrates were analyzed by ICP to obtain the equilibrium concentration (C e ).
For studying the adsorption kinetics of copper ions on hydrogels, three typical kinetic equations inclusive the pseudo-1st-and 2nd-order, and intra-particle diffusion kinetic models were explored.
The linearized pseudo-1st-order kinetic equation can be depicted as the subsequent equation: wherein q t (mg g −1 ) is the adsorption capacity at contact time (t), and q e (mg g −1 ) is the adsorption capacity at equilibrium and k 1 (min −1 ) is the pseudo-1st-order adsorption kinetic rate constant.
The linear illustration of the pseudo-2nd-order kinetic equation is measured by the following equation: wherein q e and q t (mg g −1 ) are the adsorbed Cu (II) at equilibrium point and at time t (min) and k 2 (g mg −1 min −1 ) is the kinetic rate constant of 2nd-order adsorption.
( 5) The intra-particle diffusion kinetic model was used to express the kinetic data by applying the Weber-Morris equation as follows: In Eq. ( 8), q t (mg g −1 ) is the quantity of Cu (II) adsorbed at time t, C (mg g −1 ) is a constant associated to the thickness of the bounding layer and k id (mg g −1 min −0.5 ) is the intra-particle diffusion rate constant.

Examining the Degree of Swelling of PNIPAM Hydrogel (I)
PNIPAM hydrogel (I) (0.1 g) was soaked in distilled water (10 mL) at room temperature.At varied time intervals between 5 and 240 min, swollen hydrogels were dehydrated with the filter paper and then weighed by the lab balance.The swelling amount of hydrogels (S%) was determined from Eq. ( 9): In the preceding equation, m w and m d (g) are the mass of wet sample at time t and dry sample at the beginning of the test, respectively.

Investigating the Degree of Equilibrium Swelling Ratio of PNIPAM Hydrogel (I)
PNIPAM hydrogel (I) (0.1 g) was soaked in distilled water (10 mL) at ambient temperature and retained for 2 months to assure that sample was swollen to the greatest extent.The ultimate or equilibrium amount of water absorbed over the mass (8) of dry sample is named equilibrium swelling ratio (ES%) and is figured according to Eq. ( 10): In the preceding equation, ES is the equilibrium swelling ratio and W d and W w (g) express the mass of dried and swollen hydrogels after two months, respectively.

Catalytic Reduction of 4-NP to 4-AP Using Schiff-Bases Modified PNIPAM Hydrogel Supported Copper Complex (VIII) in "On" and "Off" Mode
The catalytic activity of PNIPAM hydrogel loaded copper complex (VIII) as thermo-responsive catalyst was investigated in the catalytic reduction of 4-NP to 4-AP phenol as a model reaction.The catalyst (0.1 g), aqueous solution of 4-NP (5 mM, 5 mL), and freshly prepared NaBH 4 (50 mM, 1 mL) were mixed together and stirred at ambient temperature.The progress of the reaction was followed through UV-Vis spectrophotometer in the range of 200-400 nm wavelengths at different time intervals (0, 2 and 4 min).The experiment was repeated at 45 °C and monitored at time intervals of 5, 10, 15, 20, 25, 30, 35, 40, 45, 50 and 55 min while other conditions were constant.

Synthesis and Characterization of Cu Adsorbent Based on Thermo-Responsive Modified PNIPAM Hydrogel
Initially, some cross-linked PNIPAM hydrogels with varying percent of cross-linking agent (0.5, 1 and 2%) and LCST around 33 °C were prepared by free radical solution polymerization.In this process, NIPAM as monomer, NNMBA as cross-linking agent, AIBN as a thermal initiator and 1, 4-dioxane as solvent were used properly (Scheme 1).Percent of crosslinking is a critical point in the hydrogel synthesis since the mechanical properties and swelling amount depends strongly on the cross-link density.Among these cross-linked hydrogels (0.5, 1 and 2%), the 1% cross-linked PNIPAM with intermediate cross-link density and soft semi-solid appearance was selected, as 0.5% cross-linked PNIPAM (low cross-link density) was converted to a very soft gel with slight mechanical strength and 2% crosslinked PNIPAM (high cross-link density) was converted to the solid network with low swelling ability.The synthesized hydrogel can swell and keep a large amount of water when immersed in an aqueous solution.This stimuli-sensitive hydrogel shows abrupt changes in its swelling behavior in response to temperature as external stimuli at its LCST (~ 33 °C). Figure 1 depicted the swelled (hydrophilic) and de-swelled (hydrophobic) PNI-PAM hydrogel material (I) and schematic diagrams of the collapse process for the PNIPAM intelligent hydrogel with a rise in temperature.
The PNIPAM hydrogel (I) was characterized using FT-IR spectroscopy.The FT-IR description of compound (I) displayed the distinctive adsorption of (NH) stretching and bending of amide group (CO-NHR) at 3438 and 1541 cm −1 , respectively.The peak of the carbonyl group (C = O) vibrational band from -CONH of NIPAM is appeared at 1653 cm −1 .The signal at 2876-2973 cm −1 was related to the stretching vibrations of -CH 2 -and -CHgroups of the main chain (Fig. 2).
The sample morphology was investigated through SEM.The hydrogel was swollen to equilibrium in distilled water at ambient temperature and then freeze-dried at − 50 °C to completely remove water.Photographs of PNIPAM hydrogel (I) show a porous, hollow structure with pore size of around 2 μm and a relatively uniform distribution (Fig. 3).

Scheme 1 Depiction of PNIPAM hydrogel (I) preparation
The amino modified PNIPAM hydrogels were prepared by the reaction of PNIPAM hydrogel (I) with the excess of different diamines (i.e., hydrazine, ethylenediamine, and hexamethylenediamine) (Scheme 2).LCST of these amino modified hydrogels are increased and reached to 39 °C, 38 °C and 35 °C for compounds (II), (III) and (IV), respectively.The LCST of these compounds are increased compare to PNIPAM hydrogel (33 °C) due to the importing amino groups in their structure.On the hand LCST of these compounds are decrease by increasing the spacer arm from 0 to 6 carbon and decreasing the polarity of compounds.Figure 4 shows FT-IR spectrum of compound (III).Primary amine exhibited two peaks due to asymmetric and symmetric N-H stretching separated by around 100 cm −1 which overlap with NH stretching of amide group.Furthermore, absorption band of (C = O) carbonyl group and N-H amide bending are appeared at 1654 and 1541 cm −1 , sequentially.The signal at 2876-2973 cm −1 was associated to the methylene groups (Fig. 4).
The CHN analysis of PNIPAM (I) and amino modified PNIPAM hydrogels (II, III and IV) were represented in Table 1.The nitrogen percent was changed from 9.01% for PNIPAM hydrogel (I) to 11.55%, 12.95% and 10.21% for amino modified PNIPAM hydrogels (II), (III) and (IV), respectively.Therefore, the percent of functionalization was 12.58%, 32.32%, and 29.26% for PNIPAM hydrogel (II), (III) and (IV), respectively.As a result, we chose PNIPAM hydrogel (III) to continue our study due to the more quantity of amino content.
Furthermore, the amount of nitrogen content on the amino modified hydrogels (II, III and IV) was measured through reverse acid-base titration method and was obtained 12.2, 14.1, and 11.7 mmol/g, respectively.
Schiff-bases modified PNIPAM hydrogels (V, VI and VII) were prepared by the treatment of compound (III) with different aldehydes i.e., salysilaldehyde, thiopheneand pyridine-2-carbaldehyde in EtOH as illustrated in Scheme 3. The LCST of these Schiff-base modified   The CHN analysis of Schiff-base modified PNIPAM hydrogels (V, VI and VII) were represented in Table 2.The increment in the carbon content showed that the aldehydes are attached to the amino modified PNIPAM hydrogels successfully.In addition, among various aldehydes (i, ii and iii) salysilaldehyde (i) was converted to the corresponding Schiff-base with more imine content.
Figure 5 show the SEM pictures of Schiff-base modified PNIPAM hydrogel (V).For compound (I), the inner structure of the PNIPAM hydrogel was identified by a common sponge like large-pore morphology.On the contrary of PNI-PAM's micrograph, the SEM images of Schiff-base modified PNIPAM hydrogel (V) indicated that pores are filled, and inner structure is more compact.

Models of Adsorption Isotherm
The operation of the Schiff-base modified hydrogels (V, VI and VII) in Cu (II) elimination was explored.At first, the hydrogels were added to the aqueous solution of copper (II) acetate with varied concentrations and then, the amount of copper adsorbed on the samples was measured.The outcomes are denoted as q eq (mg g −1 ) i.e. mg of Cu (II) per gram of adsorbent at equilibrium against C eq (mg L −1 ) i.e. equilibrium concentration of Cu (II) whereas the original concentration C 0 changed from 2.5 up to 20,000 mg L −1    (2.5, 10, 50, 100, 250, 500, 800, 1500, 2000, 2500, 3000, 5000, 7000, 10,000, 15,000, 20,000 mg L −1 ).The amount of q eq increased with enhancing C 0 due to the fact that sorption equilibrium was easily driven by the increasing value of copper in solution.
To use best the adsorbents, it is crucial to study the equilibrium adsorption comprehensively and obtain the sorbent capacity.It is expressed by sorption isotherm constants whose amounts declare the surface characteristics and affinity of the sorbent.Sorption isotherms described the equilibrium dependence between sorbent and sorbate often through the value of sorbed material and that remaining in the solution at a constant temperature at equilibrium.Freundlich and Langmuir isotherms are two usual models to explain the function of adsorbents and were applied to study the experimental data.The Langmuir isotherm (Eq. 3) illustrates a homogeneous system while the Freundlich isotherm (Eq.4) depicts a heterogeneous adsorption system.In addition, Langmuir and Freundlich models revealed single and multiple layer adsorptions, respectively.
Langmuir, Freundlich and Temkin isotherm models were employed to investigate the experimental adsorption isotherm data and the fitted results of copper ions on the Schiff base functionalized hydrogels (V, VI and VII) were represented in Fig. 6.The best fitting is observed in the concentration range of 2.5-250 mg L −1 .Furthermore, The fitting parameters for the Langmuir (q m (mg.g −1 ), K L, (L mg −1 ) and R 2 ) and Freundlich (n, K f (mg g −1 ) (L mg −1 ) 1/n and R 2 ) isotherm models are expressed in Table 3.The obtained data revealed that the hydrogel (VII) gave a poorer fit compared to the hydrogels (V) and (VI) due to the lower R 2 .From the Langmuir model, the greatest amount of adsorption capacity of Cu (II) ion on the hydrogels (V), (VI) and (VII) is predicted to be around 8550, 5700 and 3132, mg g −1 , respectively.In addition, Temkin model was used to study the adsorption data experimentally.Contrary to those previous models, the Temkin isotherm assumes that the sorption free energy is dependent to the surface coverage due to the sorbent and sorbate interactions.
Schematic illustration of copper binding to the Schiffbase modified PNIPAM hydrogels (V, VI, and VII) for the forming of Cu complexes (VIII, IX, and X) is represented in Scheme 4.
To distinguish the binding behavior of hydrogel adsorbent, the desorption data of copper obtained by elution with HCl solution was investigated.The fitting of the Freundlich and Langmuir isotherm models to the desorption data of Cu (II) bound to the hydrogel was fulfilled and the graphs are represented in Fig. 7. Also, the fitting parameters are illustrated in Table 3.The profiles depicted that the Freundlich model gives the best explanation of the isotherm based on desorption data.
Lastly, for assessment the operation of adsorbents in this system, the separation factor (R L ) as an important criterion is measured according to the following equation.K L is the adsorption constant (L mg −1 ), reflecting the tendency of the absorbate to the absorbent or adsorption (11) Fig. 7 The data of desorption isotherm and the fitting of the Langmuir, Freundlich and Temkin isotherm models for (a) Schiff-base modified PNIPAM hydrogels copper complex (VIII), (b) (IX) and (c) (X) Table 3 The fitting constants of the Freundlich and Langmuir models to the adsorption and desorption data of copper ion energy.The value of R L expresses useful information about adsorption system where R L > 1, R L = 1, 0 < R L < 1, and R L = 0 means that the adsorption process is unfavorable, linear, favorable, and irreversible, respectively.The calculated R L amounts as a function of various initial Cu concentrations for (V) and (VII) are shown in Fig. 8.It was remarked that the value of R L is in the range of 0-1 and confirms the favorable process of Cu uptake by the hydrogel.R L of hydrogel (VI) is unfavorable because R L > 1 is obtained.

Kinetic Models
The impact of contact time on the adsorption of copper ion was considered at different time intervals between 30 min and 24 h at ambient temperature, whereas the initial copper ion concentration is 10 mg L −1 , adsorbent dose is 0.1 g, and the pH of the medium is 6.From Fig. 9, it was discovered that the adsorption capacity of Cu increased by developing the contact time.The adsorption capacity increased gradually and ultimately reached to the plateau state.Indeed, equilibrium adsorption was achieved after about 15 h.Increasing removal efficiency of copper ion over time can be explained by more available active sites at the primarily stages of the adsorption process and increasing swelling percentage of hydrogel that may supply more available active sites for the sorbed material.The most common kinetic models for demonstrating the kinetic of adsorption process and rate-controlling step are pseudo-first and second order and intraparticle diffusion models.Figure 10a displays the plot log (q e -q t ) versus t which declaring pseudo-first order kinetic model in the linear form, Fig. 10b depicts the plot t/q t versus t which demonstrating pseudo-second order kinetic model in the linear form and Fig. 10c illustrated the plot of q t against t 0.5 that entitled intraparticle diffusion kinetic model, respectively.As demonstrated in Fig. 10c, the plot has two different adsorption stages.The initial nearly linear stage could be associated to intra-particle diffusion and quick surface adsorption.The second nearly plateau stage could be related to the pore diffusion, where the intra-particle diffusion begins to decelerate.
In addition, Table 4 depicted the fitting constant for the 1st and 2th order and intraparticle diffusion kinetic models.The correlation coefficient (R 2 ) of the pseudo-2th order kinetic model is more closed to 1 (R 2 = 0.999), revealing that the sorption kinetic conforms the pseudo-2th order model and adsorption of Cu (II) ions onto hydrogel is a rate-controlling step.As a result, the capacity of equilibrium adsorption achieved from the experimental results (q e, exp = 7.26 mg g −1 ) is near to the estimation amount for the pseudo-2th order kinetic (q 2 = 9.259 mg g −1 ).Table 4 The Kinetic constants obtained for pseudo-1st order, pseudo-2th order, and intraparticle diffusion model for Schiff-base modified PNI-PAM hydrogel (V) Pseudo-first order Pseudo-second order Intraparticle diffusion K 1 (min −1 ) q 1 (mg g −1 ) R 2 K 2 (g mg −1 min −1 ) q 2 (mg g −1 ) R 2 K id (mg g −1 min −0.

Effect of pH
The pH is among the most crucial criterion affecting the adsorption process.In a typical methodology, the hydrogel (V) was mixed with Cu (II) solution and then pH of a solution was fixed in the range from 2 to 12 by adding drops of HCl or NaOH to the samples.The mixture of hydrogel in copper ion solution is shaked for one day to reach the adsorption equilibrium and then the adsorbents were filtered.
The supernatant was evaluated to realize the amount of copper ion.
As it can be seen from Fig. 11 with increasing pH from 2 to 6, the removal efficiency was significantly increased and then remained nearly fix at higher pH.Low adsorption capacity under acidic pH may be due to large value of H + that prevent the adsorption of Cu ions to the binding sites in solution or maybe destruction of Schiff base structure in strong acid environment.Furthermore, low decrease of adsorption capacity at a pH range of 8 to 12 was derived from hydrolysis of Cu (II) ions as expected, conducing to a decrease in the value of free copper ions.As a result, the greatest amount of adsorption capacity for the Cu (II) was achieved at pH 6.Therefore, the optimal pH value is appointed to be 6.

Swelling and Water Uptake Capacity of Hydrogel
The hydrogel ability for water uptake was investigated to figure out the diffusion of the water into the hydrogel, which is useful in the applications of heavy metal elimination.Certain amount of the adsorbent (m d ) was soaked directly in predetermined value of distilled water at ambient temperature and then at specified time intervals between 5 and 240 min, hydrogels were taken out and quickly dried with filter paper.Next, the hydrogels weighted to obtain m s and the percentage of swelling ratio of the product was measured according to Eq. 9 and then plotted as depicted in Fig. 12.In addition, for obtaining the equilibrium degree of swelling, the hydrogel was immersed in distilled water for one month and subsequently ES was calculated according to the Eq. 10 and obtained to be 1540%.
In addition, in order to compare adsorption capacity of the present work with some previous hydrogels that reported in the literature, Table 5 is introduced.It is revealed that the maximum amount of adsorption capacity for Schiff base modified PNIPAM hydrogel was around 8550 mg g −1 , which is significant as compared with some preceding reports.

Switchable Catalytic Activity of Thermo-Responsive Hydrogel Copper Complex (VIII) in the Reduction of 4-NP to 4-AP
The catalytic function and thermo-responsiveness behavior of hydrogel copper complex (VIII) was studied in the catalytic reduction of 4-NP to 4-AP using NaBH 4 reductant at two temperatures (below the LCST i.e., 25 °C and above the LCST of the catalyst i.e., 45 °C) (Fig. 13).The progress of the reaction was followed through UV-vis spectrometer during time.The catalyst performance could be evaluated from the time needed for disappearance of a peak at 400 nm relevant to 4-NP and appearance a new peak at 300 nm corresponds to 4-AP.Generally, this reaction does not happen without catalyst because of a kinetic barrier associated to the large band gap between electron donor (substrate) and acceptor (reductant).To prove this claim, a control reaction was performed without catalyst and revealed that marked change does not occur during 1 h. Figure 14 depicts the UV-vis spectra during the conversion of 4-NP to 4-AP using catalyst (VIII) at two temperatures.In the presence of the catalyst at 45 °C (Fig. 14a), the peak at 400 nm associated to 4-NP remained nearly unchanged during 1 h and therefore revealed that the reduction process did not develop.In the presence of the catalyst at 25 °C (Fig. 14b), the intensity of an absorption peak at 400 nm related to 4-NP quickly decreased and a new peak respective to 4-AP appeared at 300 nm.After a time around 4 min, the peak of 4-NP compound was no longer discerned, resulting that reduction reaction had occurred completely.
In conclusion, at temperature lower than LCST of the catalyst (25 °C), catalytic reduction performed instantaneously in just 4 min, while at temperature more than the LCST of the catalyst (45 °C), the reaction rate decreased intensely.The extreme difference of catalytic performance between 25 and 45 °C can be assign to the thermo-responsive behavior of PNIPAM.At temperature lower than LCST of PNI-PAM, hydrogen bonding between water and amide groups in PNIPAM chains caused H 2 O adsorption into the polymer matrix, expansion, and swollen coil structure of PNIPAM in solution.This hydrophilic nature of the interface due to the extended conformation of polymer chains, allowing diffusion of reagents into the nanoreactor and proceed toward copper catalyst through polymer matrix.At temperature above the LCST of PNIPAM, the hydrogen bond between H 2 O molecules and PNIPAM chain was broken, and water was escaped from the PNIPAM hydrogel.Meanwhile, the formation of intermolecular hydrogen bond in the polymer caused shrinkage and collapse of the polymer structure.Therefore, the production of hydrophobic, dense, and contracted PNIPAM structure, hindering the entrance of 4-NP into the nanoreactor in the vicinity of copper catalyst and prevent the reaction development.Consequently, the catalyst displays much better catalytic performance at 25 °C than at 45 °C.
Such temperature-responsiveness in catalytic function led to reversible on/off behavior of the catalyst.The hydrogel represents excellent stability and reusability in this reduction reaction.The reusable ability of the catalyst was evaluated by rerunning the catalytic reduction.The on-off process of recycling was repeated for at least 5 times.The catalyst indicated good catalytic activity without considerable depletion in the catalytic performance.

Conclusion
In this study, a new set of thermo-responsive adsorbent hydrogels based on PNIPAM was prepared.The prepared adsorbents showed the high removal efficiency for the elimination of copper ions from aqueous solution.The greatest amount of adsorption of Cu 2+ ions was achieved at equilibrium time of 24 h and pH 6. Adsorption isotherm considerations reveal that the Langmuir model gives a superior description of the system based on adsorption data.Also, the greatest amount of adsorption capacity for the adsorbent was estimated at 8550 mg g −1 , which is significant as compared to the latest works.The absorption kinetic follows the pseudo-second order model, demonstrating chemical sorption as the rate-determining step of adsorption mechanism.Furthermore, thermo-responsive hydrogel loaded copper (II) was applied successfully as active catalyst for catalytic reduction of 4-NP to 4-AP.At temperature less than LCST of modified PNIPAM, the reduction of 4-NP was accomplished rapidly, whereas at temperature above the LCST of catalyst, the reaction proceeds slowly.

Fig. 1 a
Fig. 1 a Swelling structure of PNIPAM (I) below the LCST and b collapsed structure of PNIPAM (I) above the LCST

Fig. 11 Fig. 12
Fig.11Effect of pH value on the Cu (II) ion adsorption process using Schiff-base modified PNIPAM hydrogel (V) (reaction condition: initial copper ion concentration = 10 mg L −1 and hydrogel mass = 0.1 g at room temperature)

Fig. 14
Fig. 14 UV-vis spectra during the conversion of 4-NP to 4-AP in the presence of the PNIPAM hydrogel Cu catalyst (VIII) at (a) 45 °C, and (b) 25 °C

Table 1
Scheme 3 Depiction of different Schiff-base modified PNIPAM hydrogels (V, VI and VII) preparation

Table 2
CHN analysis of Schiff-base modified PNIPAM hydrogel (V, VI and VII)

Table 5
Comparison of the adsorption capacity (mg g −1 ) of Schiff-base modified PNIPAM hydrogels as adsorbent with some previous hydrogels that reported in the literature